Rail butt welders are known. Such devices are generally relatively immobile and are typically maintained in stationary locations where rails to be joined are brought to the butt welder. See, for example, U.S. Pat. No. 5,270,514 which is incorporated herein by reference.
Such rail butt welders are typically constructed with two sets of conductive clamps that make contact with and pull together opposing ends of the lengths of rail that are to be butt welded. One set of clamps is relatively stationary, while the other is moveable. The moveable contact is generally provided with at least one fluid pressure actuated cylinder or ram that is operative to pull a first rail length or section (via the clamped contact) into end-to-end contact, generally termed butted relation, with a second rail section.
One or more power supplies are connected across the contacts formed by the clamps, in series across the butted joint. Activation of the power supplies results in local heating of the joint through a flashing process and ultimately in fusion of the metal forming the joint. The welding process may be divided into a number of phases. For example, a straight flashing phase can be used to burn away any projections or unevenness on either face of the butted joint. The straight flashing phase is followed by a preheat phase where a low duty cycle may be used to uniformly heat each rail end. The preheat phase is followed by a final flashing phase where the rail ends are burned off at a progressive rate to eliminate oxide inclusions and any cratering that has developed during previous phases. Finally, an upsetting and forging phase is effected wherein the contacting ends of the rails are forced (forged) together at a predetermined pressure to a point of refusal, not to a fixed distance such as a limit switch point.
During the straight flashing phase, the rate of close between the rail ends generally begins at a relatively low rate and is increased as the opposing rail ends become hot enough to sustain flashing. Current is detected and closing may be reversed if excessive current develops, indicating that the current arc has been extinguished. Control of the rate of close may be achieved by controlling the rate at which fluid flow is introduced into the actuating piston or ram that brings the rail ends together.
During the preheat phase, the rail ends are repeatedly separated until a power set point is satisfied. The total energy introduced into the rail ends is calculated per unit time to produce a constant rate of rise in rail temperature in the butted ends of the rails. The butted ends may be intentionally separated between pulses to avoid freezing of the metal, and the formation of localized short circuits. In the final flashing phase, the energy dissipated between the butted rail ends is intentionally increased. Significantly more energy is ejected from the weld area as metal splash than in further heating.
In the final step, the butted rail ends are forged (i.e., forced) together at a predetermined force that may be as high as 9000 psi. The welded joint is then quenched to achieve a desired metallurgy.
The rail butt welding process thus far described has been found to be effective in factory or plant rail production facilities for creating strings of rail where discrete lengths of rail on one or both sides of the intended butt weld are supported by roller conveyors or other supports enabling low drag longitudinal movement of the aligned rails relative to each other. The extension of the aforedescribed process to on-site butt welding of varying lengths of rail supported on or adjacent a roadbed has been inhibited by a perceived difficulty of precisely controlling the forging process.
On-site or roadbed-site butt welding of railway rails presents problems not present in butt welding lengths of rail at an off-site factory or plant production facility. For example, in the case of initial field installation of railway rails (where at least one rail is not fastened to the roadbed), the rail to be welded may vary in length from relatively short lengths, i.e., approximately 14 feet in the case of insulated joints, to lengths in excess of 2000 feet. The rail may also be supported under different conditions such as laying to the side of the right-of-way, directly on the ballast, on the track plates, or a combination of these conditions. Also, track machinery may be present either on or off the rail being welded. All of these factors establish the environmental conditions of the rail and contribute to vary the force required to move two rails together in a fashion to form a flash butt rail weld. This force requirement may be referred to as the drag force.
On-site closure welding of adjacent ends of railroad rails where both rails are fastened to the roadbed also presents problems not encountered in flash butt welding of rails in a plant production facility. During field or on-site closure welding of railroad rails to relieve stress in long sections of rail fastened to the roadbed, the welding machine must "stretch" the two rails together in order to form a flash butt railroad rail weld. The amount of stretch force required is highly environment dependent.
Accordingly, a need exists for a method and apparatus for affecting butt welding of railroad rail ends at a roadbed site and which facilitates controlled forging force flash butt welding of various length rails in initial field installation and in closure welding requiring different drag and stretch compensation.